When it comes to user-side equipment in broadband fiber access, we often see terms such as OLT, ONU, ONT, SFU, and HGU. What are they? What’s the difference?
PON Components: OLT, ONU, ONT, and ODN
A Passive Optical Network (PON) employs fiber optics and splitters to distribute data from a single source to multiple users using a point-to-multipoint topology. Unlike Active Optical Networks (AON), PONs operate solely in the optical domain, without requiring electrical power. The two primary PON standards are Gigabit Passive Optical Network (GPON) and Ethernet Passive Optical Network (EPON). In an EPON (GEPON) system, components include the Optical Line Terminal (OLT) at the provider’s central office, Optical Network Units (ONUs) or Optical Network Terminals (ONTs) at user endpoints, an Optical Splitter (SPL), and an Optical Distribution Network (ODN) for data transmission between the OLT and ONU/ONT.
What is OLT?
An Optical Line Terminal (OLT) is a crucial device used in optical fiber networks to facilitate signal transmission. It serves as the starting point for Passive Optical Networks (PONs) and connects to an aggregation switch through Ethernet cables, playing a pivotal role in PON setups.
OLT Equipment Components
Typically, OLT equipment comprises a rack, Control and Switch Module (CSM), EPON Link Module (ELM), also known as the PON card, redundancy-protected -48V DC power supply modules or a single 110/220V AC power supply module, as well as fans. The PON card and power supply support hot-swapping, while another module is internally integrated. OLT functions in two directions: upstream (receiving and distributing various data and voice traffic from users) and downstream (receiving data, voice, and video traffic from the metro or long-haul network and transmitting it to all ONT modules on the Optical Distribution Network (ODN)).
OLT Functions
The OLT device plays a central role in Ethernet Passive Optical Networks (EPON). It acts as a versatile platform, supporting both IP services and traditional TDM services simultaneously. Typically situated at the edge of the Metropolitan Area Network (MAN) or the exit of the community access network, the OLT converges access services and transmits them to the IP network.
EPON’s passive optical network system offers flexibility, connecting multiple terminals within a 20-kilometer radius to create a network. This system accommodates various business models, adapts to different working environments, and offers FTTx solutions to users. In addition to service aggregation, the OLT serves as a centralized network management platform. It enables device-based network element management, service-based security management, and configuration management. This includes monitoring, managing devices and ports, provisioning services, monitoring user status, and allocating bandwidth according to Quality of Service (QoS) and Service Level Agreement (SLA) requirements.
The OLT’s key functions also include sending Ethernet data to ONUs in broadcast mode, initiating and controlling the ranging process, and bandwidth allocation for ONUs, which regulates when ONUs send data and the size of their sending windows.
OLT Application
OLT devices collaborate with various types of ONUs to establish diverse access networks such as FTTC, FTTH, FTTO, FTTM, and more. On one hand, they aggregate signals carrying various services at central offices, transmitting them to end-users on the access network in specific signal formats. On the other hand, signals from end-users are routed to different service networks based on their service types. This configuration enables a PON to serve multiple households, providing various services such as IPTV, VOIP, and IP Cameras.
What is ONU and what is ONT?
The main application types of broadband optical access include: FTTH, FTTO and FTTB, and the user-side equipment has different forms under different application types.
The user-side equipment of FTTH and FTTO is used by a single user, which is called ONT (Optical Network Terminal).
The user-side equipment of FTTB is shared by multiple users, called ONU (Optical Network Unit).
The user mentioned here refers to the user independently billed by the operator, not the number of terminals used. For example, the ONT of FTTH is generally shared by multiple terminals in a home, but only one user can be counted.
ONT and ONU both refer to the consumer end equipment in an optical fiber to the home (FTTH) communication link. ONU converts optical signals transmitted via fibers to electrical signals. These electrical signals are then sent to individual subscribers. In general, there is a distance or other access network between ONU and end user’s premises.
Types of ONTs
ONT (Optical Line Terminal) is the modem, which is divided into SFU (Single Family Unit), HGU (Home Gateway Unit) and SBU (Single Business Unit).
- SFU
SFU generally has 1 to 4 Ethernet interfaces, 1 to 2 fixed telephone interfaces, and some models also have cable TV interfaces. The SFU does not have the function of home gateway, the terminal connected through the Ethernet port can dial up to the Internet, and the remote management function is weak. The modem used in the early days of FTTH (Fiber to the Home) belongs to SFU and is rarely used at present.
- HGU
The modems equipped by FTTH users opened in recent years are all HGUs. HGU usually comes with wifi and has a USB interface.
- SBU
SBU is mainly used for the access of FTTO users, generally with Ethernet interface, some models have E1 interface, fixed telephone interface, or with wifi function. Compared with SFU and HGU, SBU has better electrical protection performance and higher stability, and is also commonly used in outdoor occasions such as video surveillance.
Types of ONUs
The MDU is mainly used for the access of multiple residential users under the FTTB application type, and generally has at least 4 user-side interfaces, usually 8-way, 16-way, 24-way FE or FE+POTS (fixed telephone) interface.
The MTU is mainly used for the access of multiple enterprise users or multiple terminals in the same enterprise in the FTTB scenario. In addition to the Ethernet interface and the fixed telephone interface, the MTU may also have an E1 interface. The appearance and function of the MTU are generally the same as those of the MDU, but the electrical protection performance is better and the stability is higher. With the popularity of FTTO, the application scenarios of MTU are getting smaller and smaller.
ONT vs ONU
Both Optical Network Unit ONU and Optical Network Terminal ONT are terminal devices to connect users in a passive optical network. Then, what is the difference?
- Deployment
ONT: An ONT is located at the customer premise (residential or commercial).
ONU: ONU can be placed in either the user’s home or the provider’s central office. - User
ONT: A single user uses the user-side equipment of FTTH and FTTO.
ONU: Multiple users share the user-side equipment of FTTB. - Categories
ONT: SFU, HGU, and SBU belong to ONT.
ONU: MDU and MTU belong to ONU. - Function
The ONU usually communicates with an ONT, which may be a separate box that connects the PON to TV sets, telephones, computers, or a wireless router.
An ONU converts optical signals to electrical signals via a fiber cable. An ONU organizes and optimizes different types of data coming from customers to efficiently send it upstream to the Optical Line Terminal OLT (located at an ISP’s central hub).
What is ODN?
The Optical Distribution Network (ODN) is a vital component of the PON system, serving as the optical transmission medium that connects ONUs to OLTs over distances of 20 km or more. It encompasses various elements, including fiber optic cables, connectors, passive optical splitters, and auxiliary components. The ODN comprises five distinct segments:
- Feeder Fiber: Originating from the optical distribution frame (ODF) in the central office telecommunications room, feeder fiber extends to the optical distribution point, covering long-distance connections.
- Optical Distribution Point: This segment bridges the gap from the feeder fiber to the distribution fiber, providing optical fibers for adjacent areas.
- Distribution Fiber: Extending from the optical distribution point to the optical access point, distribution fiber distributes optical signals to nearby regions.
- Optical Access Point: At this stage, the optical signals are made available for connection to user terminals (ONTs), facilitating the optical fiber drop into users’ homes.
- Drop Fiber: The final segment connects the optical access point to ONTs, completing the optical fiber connection to individual user residences.
The ODN’s quality and reliability are critical, as it forms the essential path for PON data transmission. The performance of the ODN directly impacts the overall performance, reliability, and scalability of the PON system.
Summary
Broadband optical fiber access mainly adopts PON (passive optical network) technology. When the specific form of user-side equipment is not distinguished, the user-side equipment of the Passive Optical Network PON system can be collectively referred to as ONU.
ONU, ONT, SFU, HGU… These all describe the user-side equipment of broadband access from different perspectives, and the relationship between them is shown in the following figure. Contact us ([email protected]) if you have any questions.
